1. Field of the Invention
The invention relates generally to settable wellbore fluids. More particularly, the invention relates to methods of using these settable wellbore fluids in drilling applications.
2. Background Art
During the drilling of a wellbore, various fluids, often called “mud,” are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, the drilling fluid acts to lubricate and cool rotary drill bits, to prevent blowouts by providing hydrostatic pressure to balance any high-pressure formation fluids that may suddenly enter the wellbore, and to remove cuttings from the wellbore.
One common step during the construction of the wellbore involves placing a pipe string, e.g., casing, into the wellbore. Well casings of various sizes are typically used, depending upon depth, desired hole size, and geological formations encountered. The casing serves several functions, including providing structural support to the wellbore to prevent the formation walls from caving into the wellbore. The casing is typically stabilized and bonded in position within the wellbore. However, because drilling fluids are generally not settable (i.e. they don't develop compressive strength or create a solid bond with casing and formation surfaces), a portion of the drilling fluid is typically removed from the wellbore so that the casings may be set in place by a primary cementing operation.
Primary cementing operations fill at least a portion of the annular space between the casing and the formation wall with a hydraulic cement composition. The cement composition may then be allowed to solidify in the annular space, thereby forming an annular sheath of cement. The cement barrier is desirably impermeable, such that it will prevent the migration of fluid between zones or formations previously penetrated by the wellbore.
When completed, the well's annular space will contain a solidified cement section that bonds the casing, provides support to the casing string and isolates permeable zones, a section that may contain chemical spacers and interfaces of fluids used to place the cement including mud, water, weighting agents, surfactants, salts and cement, and a section that contains drilling mud that was used to drill the interval which was just cased, as described in U.S. Pat. Nos. 4,671,357 and 6,283,213. Over the life of the well these fluids may degrade and separate into the individual components, such as water, solids and oil. This separation may result in a reduction of hydrostatic pressure near the upper edge of the cement.
Problems often encountered in wellbores include those related to the build up of annular casing pressure and the migration of fluids. Pressure often develops in the annular space between casings of differing size because leaks between strings of casing, tubing leaks, packer leaks, wellhead packoff leaks, and substandard cementing jobs. Cracks and fissures in the annular cement barrier may be caused by factors related to the cement composition, cement spacer, chemistry, displacement efficiency, thermal stress (casing expansion/contraction), hydraulic stress, and compaction. Cracks and poor cement bonds may provide a pathway through which high pressure fluids can migrate. Fluid migration can lead to wellbore failure or excessive annular casing pressure build up.
Annular casing pressure build up can occur in wellbores that have been drilled with water or invert emulsion based fluids. Invert emulsion based fluids may show a greater tendency toward annular casing pressure build up due to the inherent incompatibility between oil-based fluids and water-based displacement and cement fluids. These incompatibilities may result in poor cement bond or the degradation of invert emulsion-based fluid suspension characteristics as the fluids synerese into the base components of oil, emulsifiers, solids, and brine. In addition, greater gas solubility in the oil-based fluid may contribute to annular casing pressure development. The predominant suspension properties of an invert emulsion fluid center around the emulsion, whereas for a water-based mud, viscosifying solids and/or polymers are the primary suspending agents.
To remediate and control annular pressure, a relatively heavy liquid is typically pumped into the annular space at the upper end of the well, to displace lighter liquids. However, these procedures are often very costly and do not always give the desired results.
Accordingly, there exists a need for a wellbore that has an impermeable barrier to prevent the build-up of annular casing pressure and the migration of fluids.